The present invention relates to an implant, system and method for treating a disorder of the nervous system in a subject. The method involves using passive electrical conductors which route electrical current to electrically stimulate a target body tissue to either activate or block neural impulses depending upon the frequency and the disorder to be treated.
Nerve cells consist of an axon for transmitting action potentials or neural impulses, and dendrites for receiving such impulses. Normally, nerves transmit action potentials from the impulse-sending axon of one nerve cell to the impulse-receiving dendrites of an adjacent nerve cell. At synapses, the axon secretes neurotransmitters to trigger the receptors on the next nerve cell's dendrites to initiate a new electrical current.
In some pathological states, transmission of action potentials is impaired; thus, activation of neural impulses is required to restore normal functioning. Electrically-excitable bodily tissues such as nerves and muscles may be activated by an electrical field applied between electrodes applied externally to the skin. Electric current flows through the skin between a cathode electrode and an anode electrode, eliciting action potentials in the nerves and muscles underlying the electrodes. This method has been used for many years in different types of stimulators, including transcutaneous electrical nerve stimulators (TENS) which relieve pain, therapeutic electrical stimulators which activate muscles for exercise purposes (Vodovnik, 1981), functional electrical stimulators which activate muscles for tasks of daily life (Kralj et al., 1989); U.S. Pat. No. 5,330,516 to Nathan; U.S. Pat. No. 5,562,707 to Prochazka et al.) and stimulators that promote regeneration of damaged bones.
In other pathological states, action potentials are transmitted which do not serve a useful purpose; hence, blocking of unnecessary neural impulses is required to restore normal functioning. It has been reported that high-frequency stimulation can produce temporary reversible blocks of nerve axons (Solomonow et al., 1983; Tai et al., 2004; Bhadra and Kilgore, 2005). Generally, the frequency range is between 500 and 30,000 Hz.
Stimulation of nerves to either active or block neural impulses is typically achieved with the use of an implanted stimulator (also known as a neural prosthesis or neuroprosthesis) (Peckham et al., 2001; Horch and Dhillon, 2004). Neural prostheses have been developed to restore hearing, to restore movement in paralyzed muscles, to modulate activity in nerves controlling urinary tract function and to suppress pain and tremor. In some cases, neural prostheses are designed to inhibit or suppress unwanted neural activity, for example to block pain sensation or tremors. However, all neural prostheses intended for long-term use require the implantation of a stimulator that contains electronic components and often a battery. In the case of pain and tremor suppression, the activated nerves reflexly inhibit the activity of neural circuits within the central nervous system. This indirect mode of reducing unwanted neural activity is sometimes called neuromodulation (Landau and Levy, 1993; Groen and Bosch, 2001).
Surface electrical stimulators are used reflexly for example to reduce spastic hypertonus (Vodovnik et al., 1984; Apkarian and Naumann, 1991). A disadvantage of stimulation through electrodes attached to the body surface is that many non-targeted tissues may be co-activated along with the targeted tissues. This lack of selectivity often causes unwanted sensations and/or unwanted movements. Furthermore, tissues that lie deep within the body are difficult or impossible to stimulate adequately, because most of the electrical current flowing between the electrodes flows through tissues closer to the electrodes than the targeted tissues. Selectivity may be improved by implanting wires within the body that route electrical current from a stimulator to the vicinity of the targeted tissues. This method is used in cardiac pacemakers (Horch et al., 2004), dorsal column stimulators (Waltz, 1997), deep brain stimulators (Benabid et al., 1987) and sacral root stimulators (Brindley et al., 1982). Cuffs containing the uninsulated ends of the wires may be placed around peripheral nerves to restrict most of the current to the vicinity of the nerve and limiting the spread of current to surrounding tissues, thereby improving selectivity (Haugland et al., 1999). Generally when wires are implanted, the stimulators, complete with an energy source, are also implanted (Strojnik et al., 1987). Implanted stimulators are expensive and often require a controller and/or power source external to the body. Batteries within the implanted stimulators need periodic replacement, entailing surgery.
In a minority of cases, stimulating wires are implanted in bodily tissues and led through the skin (percutaneously) to a connector attached to the surface of the body, to which an external stimulator is attached (Peckham et al., 1980; Handa et al., 1998; Shaker and Hassouna, 1999; Yu et al., 2001). External stimulators are much less expensive than implanted stimulators, but the percutaneous wires provide a conduit for infection and therefore require daily cleaning and maintenance. This has generally limited the use of percutaneous electrodes to short-term applications. There is a need for a system which overcomes such problems and has the capability of activating or blocking nerve impulses depending upon the disorder to be treated.